Project Summary/abstract: Atrial fibrillation (AF) is the most common cardiac arrhythmia and estimated to account for approximately 15% of all strokes in the United States. Studies have shown that strokes in these patients seem to be caused by peripheral thromboembolisms which form in the left atrial appendage and travel to the brain. Anticoagulation has been shown to be highly effective at preventing thromboembolism formation, but is associated with greater monitoring costs and bleeding risks. Clinicians lack non-invasive diagnostic techniques that can reliably assess atrial hemodynamics, and current stroke prevention management relies on empirically determined risk scores that have been shown to have low predictive value. 4D flow magnetic resonance imaging provides time-resolved, three directional velocity encoding, but is currently limited by long scan times and inadequate resolution. In addition, the beat-to-beat variation of AF has not been considered in current MRI protocols that collect data over multiple cardiac cycles. This project aims to develop a 4D flow MRI protocol that incorporates improved temporal-spatial resolution, decreased scan time, and real-time MRI to more comprehensively assess atrial hemodynamics in AF patients. We will develop this protocol, assess repeatability and accuracy, and apply the protocol to a small study of AF patients and healthy volunteers.